1. Field of the Invention
The present invention relates to mass flow control devices, or mass flow controllers, used in industrial gas delivery systems. More specifically, the invention relates to automatic pressure compensation to maintain a pressure differential across a mass flow controller, within predetermined limits.
2. Description of Related Art
Mass flow controllers (MFCs) are commonly employed in semiconductor manufacturing facilities to control the mass flow rates of various gases required to be supplied to pieces of manufacturing equipment. A high degree of precision is required for MFCs used in such service, as accurate control of the gas delivery is very important in maintaining the quality and consistency of the semiconductor elements produced by the equipment.
One of the characteristics of the operation of a mass flow controller is that a differential pressure is created across the MFC between the inlet or supply line side and the outlet side of the MFC, as a result of the MFC operating to maintain a substantially constant mass flow rate for the gas exiting the MFC. A major problem with MFCs that have been developed, or have been proposed for development, in the past has been that reliable operation of the MFC is not obtained due to inadequate control over the differential pressure established across the MFC. Generally, such control has required manual adjustments to the system to return the pressure differential to a stable operational value.
U.S. Pat. No. 5,129,418, issued to Shimomura et al. presents one example of a mass flow controller design which attempts to achieve improved performance and operation by monitoring operational conditions other than the flow rate, such as pressure and temperature, at designated places in the system, in order to provide output data to an operator. It is suggested that the operator would be able to troubleshoot or perform certain diagnostics while the MFC is in operation using this design. The disclosed advantage of this system is that abnormalities in operation can be found prior to the flow rate being disturbed to facilitate the planning of maintenance or component trade outs.
Other patents directed to mass flow controllers have adopted microprocessor control of the flow regulation of the MFC. Generally, such patents have employed the microprocessor control in order to implement fuzzy logic or other artificial intelligence controls for the controller.
Notwithstanding the existence of these various microprocessor-controlled systems, there continues to exist a need for a mass flow controller having increasingly reliable and stable operation. More specifically, a need exists for a mass flow controller device that does not require manual control of a pressure regulator, the output of which serves as the input to the MFC, when the MFC output becomes unstable.
It is therefore a principal object of the present invention to provide a mass flow control device having automatic adjustment of the differential pressure across the MFC in order to achieve a more stable output from the mass flow controller.
It is an additional principal object of the present invention to provide a mass flow control device in which an output signal of the mass flow control device is monitored and analyzed to determine the stability of the signal, and in which an automated control system adjusts the device as necessary to maintain a stable output.
It is a further important object of the present invention to provide a mass flow control device having means for maintaining a temperature of an incoming gas at or near a temperature used in calibrating the mass flow controller.